1. Field of the Invention
This invention relates generally to atomic-fluorescence and atomic-absorption spectroscopy; and more particularly to improved apparatus and methods for disassociating solid materials such as metal or powdered rock into free atoms--and for bringing such free atoms conveniently into a light path and precisely, accurately measuring their absorption or fluorescence.
Such measurements are useful for chemical analysis. Certain portions of the invention are useful in supplying free atoms for any purpose, but particularly in replacing the burner of a typical atomic-absorption spectrophotometer so that solid samples may be analyzed directly without the sample dissolution that historically has been required.
2. Prior Art
A commonly used instrumental analysis method for elemental analysis of various materials, for trace and minor elements, is the atomic-absorption method.
FIG. 15 represents schematically the usual instrumentation, consisting of apparatus such as a nebulizer 1 for generating a vapor, a burner assembly 2 to disassociate into free atoms 4 the vapor delivered by the nebulizer, a hollow-cathode lamp 3 or like source of monochromatic light which is directed through the atomized vapor 4, and a device 5 for isolating and measuring the intensity of that fraction of the monochromatic light that passes through the atomized vapor.
Quantitative measurement of elements present in the vapor, and thus of the sample from which the vapor was derived, is made by comparing the intensity of the monochromatic light characteristic of an element after absorption in the burner flame to the unabsorbed intensity of the light source.
Although it is a highly sensitive analytical method, atomic-absorption spectrometry has suffered from two major handicaps:
(a) With one exception outlined below, it has generally been considered necessary to prepare samples in liquid form, for presentation to the burner flame. This has required that solid material such as metals, rock, etc. be dissolved in appropriate acid solutions--entailing much time and effort.
Sputtering of free atoms in a gaseous glow discharge has been previously demonstrated, but has been practical only for atomic-emission elemental analysis and for other applications such as coating of thin metal films on a substrate.
The exception is a development project of the Australian Scientific and Industrial Research Organization (CSIRO), which did include attempts to use the sputtering process for atomic-absorption or atomic-fluorescence analysis, as reported in Analytical Chemistry 48 13, November 1976 (page 1926), and Australian Pat. No. 482,264.
The reported performance data from CSIRO--that is, the sputtering rate, sensitivity, stability, dynamic range and speed of analysis--indicate that without significant improvements in these parameters the technique was not fully practical.
(b) The inherent nature of the atomic-absorption process provides a linear relationship between concentration and measured absorbance over only a very small range of concentration. Because of this, dynamic range is limited to about two orders of magnitude.
In many cases this requires successive dilutions of the same sample, and multiple analyses, to cover the dynamic range required.
The CSIRO apparatus appears in FIGS. 16 through 18. This device replaces only the nebulizer 1 and burner 2 of FIG. 15, being employed with a generally comparable source, monochromator, and imaging system (not illustrated in FIGS. 16 through 18).
As shown in FIG. 16, the CSIRO device includes a gas-tight chamber 11, with optical-quality end windows 12 for passage of the measurement light beam generally along a central path 18. (The barrel of the chamber 11 is illustrated broken away as at 13 and 14 to permit showing the apparatus at a fairly large scale.) A pumping outlet 15 is provided for evacuation of the chamber 11, and thereafter for continuous drawing-off of the working gas. An anode 16 is sealed through the chamber wall for formation of a glow discharge in the chamber.
Serving as cathode for that discharge is the surface 41 of the specimen or sample 24. The sample 24 is accessible for this purpose through the side of the chamber 11 opposite the pump 15 and anode 16. Positive ions of the working gas are accelerated through the "dark space" or "cathode fall" region of the glow discharge toward the negatively charged active surface 41 of the sample 24. These positive ions energetically bombard the surface 41 and thereby dislodge or "sputter" atoms from that surface into the chamber.
Behind the sample 24 is a water-jacketed cooling member 25. Typically this member 25 is in intimate thermal and electrical contact with the sample 24, simultaneously drawing off heat generated by the glow discharge and establishing the cathodic potential of the sample 24. (Thus in some literature the member 25 may be referred to as the "cathode.")
The forward surface 41 of the sample is pressed or otherwise sealed against its port 17, through the intermediary of a hollow silica annulus 19 (FIGS. 16 through 18). An inert working gas such as argon is introduced into the chamber 11 through a supply tube 21 and through the annulus 19. The supply tube 21 enters the chamber at any convenient point 22 in the wall and exits at a point 24 that is quite near the sample, and in fact is aligned with a mating port 24' in the annulus 19 (see FIG. 17).
The annulus 19 directs the gas supply directly into the sample region and helps confine the discharge. As seen in FIG. 17, the annulus is made in two parts 26, 33 to permit provision of an annular manifold cavity 27 that communicates with the port 24'. A narrow slit 28 completely around the inside diameter 29, 32 of the annulus directs a sheath of high-velocity gas (see FIG. 18) from the manifold 27 into the region in front of the sample surface 41.
Sputtered sample atoms from the sample surface 41 move into the absorption zone--i.e., along the optical path 18--partially carried along in this gas stream and partially through the diffusion process.
The shallow recess 31 at the sample surface 41 prevents electrical breakdown at the annulus, because the recess is shallower than the length of the "dark space" or "cathode fall" region of a glow discharge. The annulus consequently is sometimes instead known as a "discharge arrestor."
If gas velocity at the mouth of the recess is not high enough, the shallow recess 31 tends to clog rapidly: diffusion of sputtered atoms into this region builds up a mound of previously sputtered sample material 43 (FIG. 18) on the sample 41, quickly leading to shorting-out of the dark-space region. This "short circuit" extinguishes the glow and damages the annulus. To keep this system operating, therefore, the annulus must be cleaned frequently.
Similarly diffusion of atoms in the measurement chamber, though essential for atomic-absorption measurements, causes problems: the diffusing atoms are continuously deposited on surfaces of the apparatus, diminishing the supply of free atoms for measurement purposes. Worse yet, some of the atoms are deposited on the optical windows, obscuring the measurement light beam and progressively degrading the signal-to-noise ratio of the measurement.
Yet another source of measurement imprecision arises from the surface character of typical specimens. In previous attempts to use sputtering techniques to provide atoms for absorption or fluorescence analysis, it has been awkward to obtain a representative sampling from the bulk of the specimen material. It has been found necessary to "sputter through" surface imperfections, surface chemical compounds such as oxides, and other surface phenomena to reach atoms free of these effects.
Prior workers have accomplished this by operating the sputtering discharge for a relatively long time before taking measurements considered to represent the bulk material. Preliminary measurement values can be monitored during this protracted cleaning, to assess the progress of the cleaning operation itself: representative bulk material has been reached when the observed readings come to equilibrium. This approach results in prohibitively long analysis times.